The long-term goal of the proposed research is to contribute to an understanding of the molecular mechanisms involved in the replication of chromosomes in eukaryotic cells, particularly the mechanisms responsible for regulating DNA replication during the cell cycle. This goal is a fundamental aspect of the more general objective of the Program to understand the basic mechanisms that control cellular growth and how these mechanisms are perturbed neoplastic and virus-transformed cells. Our approach to this problem involves biochemical and genetic analysis of two experimental systems. First, we will continue our studies of the replication of the simple chromosome of the DNA tumor virus, SV4O. We plan to focus our efforts on the initiation of SV4O DNA replication in a mammalian cell-free system with particular emphasis on the role of phosphorylation of viral and cellular proteins in controlling the initiation process. Second, we will employ the fission yeast, Schizosaccharomyces pombe as a model system to explore the mechanisms involved in initiating DNA replication at cellular origins. We plan to define the essential genetic elements required for origin function and to identify and characterize the cellular initiator proteins that interact with such elements.